将Django模型移动到另一个应用程序，该应用程序是另一个模型的父模型

Question

我必须在两个不同的应用程序（如app1和app2）中使用django模型，在app1中，我得到了Model1，在app2中，我得到了BaseModel，而Model1就是这样

class Model1(BaseModel):
...

Model1和BaseModel在一个应用程序中，但是我将Model1移至app2，现在我也想将BaseModel也移至app2。 我的问题是，当我尝试将BaseModel移至app2时，出现此错误：

Cannot resolve bases for [<ModelState: 'app1.model1'>]
This can happen if you are inheriting models from an app with migrations (e.g. contrib.auth)
 in an app with no migrations; see https://docs.djangoproject.com/en/2.1/topics/migrations/#dependencies for more

我的工作很简单：

1. 我编写了将BaseModel表重命名为app2_basemodel的迁移，然后编写了在App2中创建模型的迁移
2. 我创建迁移以更改用于继承的字段basemodel_ptr
3. 我将BaseModel代码移动到app2并通过从app1迁移来删除BaseModel

此方法适用于移动Model1，但是当我尝试移动此基本模型时，出现此错误。

我非常感谢您提供的帮助，包括采取其他任何方法来实现将BaseModel移至app1的重构思路。

Answer 1

Django并未为此提供内置的迁移操作，但是您可以通过将多个迁移操作拼凑在一起来实现此目的。

迁移不仅会更改数据库，而且还会保留所有模型的状态。 您必须确保状态与数据库保持同步，这会使事情变得有些复杂。 关键是使用migrations.SeparateDatabaseAndState 。

假设您具有以下模型定义：

# app1/models.py
from django.db import models

class BaseModel(models.Model):
    base_field = models.CharField(max_length=64)

# app2/models.py
from django.db import models
from app1.models import BaseModel

class Model1(BaseModel):
    model_field = models.CharField(max_length=64)

您想迁移到此：

# app1/models.py empty
# app2/models.py

from django.db import models

class BaseModel(models.Model):
    base_field = models.CharField(max_length=64)

class Model1(BaseModel):
    model_field = models.CharField(max_length=64)

您必须创建三个迁移：

• 在App1中：将app1.BaseModel的表从app1_basemodel重命名为app2_basemodel 。 这还需要调整basemodel_ptr列上的外键约束。
• 在App2中：添加app2.BaseModel并使用app2.BaseModel作为基础模型重新创建app2.Model1 。 这些更改仅针对迁移状态完成，而不涉及数据库！
• 在App1中：从迁移状态中删除app1.BaseModel 。 同样，没有数据库更改。

这是代码中的样子：

# app1/migrations/0002_rename_basemodel_table.py
from django.db import migrations, models


class Migration(migrations.Migration):
    atomic = False
    dependencies = [
        ('app1', '0001_initial'),
    ]
    operations = [
        migrations.AlterModelTable(
            name='BaseModel',
            table='app2_basemodel'
        ),
    ]

# app2/migrations/0002_change_basemodel.py
from django.db import migrations, models
import django.db.models.deletion


class Migration(migrations.Migration):

    dependencies = [
        ('app2', '0001_initial'),
        ('app1', '0002_rename_basemodel_table')
    ]

    state_operations = [
        migrations.CreateModel(
            name='BaseModel',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('base_field', models.CharField(max_length=64)),
            ],
        ),
        migrations.DeleteModel(
            name='Model1',
        ),
        migrations.CreateModel(
            name='Model1',
            fields=[
                ('basemodel_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='app2.BaseModel')),
            ],
            bases=('app2.basemodel',),
        ),
    ]
    database_operations = [
    ]
    operations = [
        migrations.SeparateDatabaseAndState(
            database_operations,
            state_operations
        )
    ]

# app1/0003_remove_basemodel.py
from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('app1', '0002_rename_basemodel_table'),
        ('app2', '0002_change_basemodel')
    ]

    state_operations = [
        migrations.DeleteModel(
            name='BaseModel',
        ),
    ]
    operations = [
        migrations.SeparateDatabaseAndState(
            database_operations=None,
            state_operations=state_operations
        )
    ]

显然，您必须调整这些迁移以反映您的实际模型。 恐怕如果您还有其他与Model1相关的模型，这可能会变得更加复杂。

免责声明：我已经使用SQLite和PostgreSQL对它们进行了测试，但使用风险自负！ 在生产数据上运行备份之前，请确保已备份。

之前：

$ python manage.py dbshell
SQLite version 3.19.3 2017-06-27 16:48:08
Enter ".help" for usage hints.
CREATE TABLE IF NOT EXISTS "app1_basemodel" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "base_field" varchar(64) NOT NULL);
CREATE TABLE IF NOT EXISTS "app2_model1" ("basemodel_ptr_id" integer NOT NULL PRIMARY KEY REFERENCES "app1_basemodel" ("id") DEFERRABLE INITIALLY DEFERRED, "model_field" varchar(64) NOT NULL);
...

$ python manage.py shell
Python 3.7.0 (default, Aug 22 2018, 15:22:33)
[Clang 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from app2.models import Model1
>>> Model1.__bases__
(<class 'app1.models.BaseModel'>,)
>>> Model1.objects.create(base_field='a', model_field='A')
<Model1: Model1 object (1)>
>>> Model1.objects.create(base_field='b', model_field='B')
<Model1: Model1 object (2)>
>>>

后：

sqlite> .schema
...
CREATE TABLE IF NOT EXISTS "app2_basemodel" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "base_field" varchar(64) NOT NULL);
CREATE TABLE IF NOT EXISTS "app2_model1" ("basemodel_ptr_id" integer NOT NULL PRIMARY KEY REFERENCES "app2_basemodel" ("id") DEFERRABLE INITIALLY DEFERRED, "model_field" varchar(64) NOT NULL);
...

>>> from app2.models import Model1
>>> Model1.__bases__
(<class 'app2.models.BaseModel'>,)
>>> for obj in Model1.objects.all():
...  print(obj.base_field, obj.model_field)
...
a A
b B

或者，您可以考虑编写自定义迁移操作 。

Answer 2

我试图从状态中删除model1，这起作用了这是我为使其工作而做的事情（某些名称与我所写的内容有所不同）：

1. 从状态中删除model1

    state_operations = [migrations.DeleteModel('Model')] operations =[migrations.SeparateDatabaseAndState(state_operations=state_operations)] 

2. 重命名BaseModel表（您需要为SQLite设置atomic = True）

    operations = [ migrations.AlterModelTable( name='basemodel', table='first_basemodel', ), 

   ]

3. 将模型移至第一个应用程序

    def update_contentypes(apps, schema_editor): ContentType = apps.get_model('contenttypes', 'ContentType') db_alias = schema_editor.connection.alias qs = ContentType.objects.using(db_alias).filter(app_label='second', model='basemodel') qs.update(app_label='first') def update_contentypes_reverse(apps, schema_editor): ContentType = apps.get_model('contenttypes', 'ContentType') db_alias = schema_editor.connection.alias qs = ContentType.objects.using(db_alias).filter(app_label='first', model='basemodel') qs.update(app_label='second') class Migration(migrations.Migration): dependencies = [ ('first', '0002_delete_model_from_state'), ('second', '0002_auto_20181218_0717') ] state_operenterations = [ migrations.CreateModel( name='BaseModel', fields=[ ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')), ('name', models.CharField(max_length=200)), ], ), ] database_operations = [ migrations.RunPython(update_contentypes, update_contentypes_reverse), ] operations = [ migrations.SeparateDatabaseAndState( state_operations=state_operations, database_operations=database_operations ), ] 

4. 从状态中删除app2中的BaseModel

    state_operations = [ migrations.DeleteModel('BaseModel'), ] operations = [ migrations.SeparateDatabaseAndState(state_operations=state_operations) ] 

5. 移动代码并添加回Model1

    state_operations = [migrations.CreateModel( name='Model', fields=[ ('basemodel_ptr', models.OneToOneField(auto_created=True, on_delete=models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='first.BaseModel')), ('title', models.CharField(max_length=200)), ], bases=('first.basemodel',),),] operations = [migrations.SeparateDatabaseAndState(state_operations=state_operations)] 

该解决方案为我工作，并且在SQLite和PSQL上测试没有任何数据丢失。